Stem cell factor (SCF) is a growth factor critical for normal hematopoiesis.. The receptor for SCF is Kit, a receptor tyrosine kinase (RTK). Activating mutations in Kit are associated with mastocytosis, leukemia, germ cell tumors and gastrointestinal stromal cell tumors. Because of the importance of Kit in normal hematopoiesis, as well as its role in human disease, the objective of this project is to understand the mechanism of action of both normal and oncogenic forms of this receptor in hematopoietic progenitor cells and mast cells, two important target populations of SCF. One specific aim of this project is to define the biochemical mechanism of action of wild-type Kit. Specifically, we have focused on the role of the JAK/STAT pathway and Src family members in SCF-mediated responses. We have previously shown that Lyn is required for progenitor and mast cells to respond normally to SCF. Our recent studies suggest Lyn contributes to SCF-mediated responses through activation of Jnks, PI3 kinase and Stats. Work is in progress to determine if this occurs through direct interaction of Lyn with these signaling components or through Lyn-dependent phosphorylation of the relevant docking sites on Kit. Although the JAK/STAT pathway is critical in responses mediated by cytokine receptors, little is known about the role of this pathway in RTK-mediated responses. Using hematopoietic cells derived from JAK2-deficient embryos we have demonstrated that JAK2 contributes to SCF-mediated proliferation of primary progenitor cells. These are the first studies to demonstrate that a JAK family member contributes to responses mediated by a RTK. A second specific aim of this project is to delineate signal transduction pathways mediating cellular transformation by D816V Kit, an oncogenic mutant found in patients with mastocytosis, leukemia and germ cell tumors. Last year we published work demonstrating that constitutive association of activated PI3 kinase with D816V Kit is required for tumorigenicity of this mutant. This year we have shown that PI3 kinase is necessary for transformation of hematopoietic cells in vitro and in vivo. Using a murine model of mastocytoma we have shown that treatment with a PI3 kinase inhibitor slows the growth of tumors generated by the Kit mutant. Studies are also underway to identify compounds that directly inhibit the growth of the Kit catalytic domain mutant. These data could have important implications in designing treatment strategies of diseases associated with this mutation, such as mastocytosis and leukemia.